1. Field of the Invention
The present invention relates to a process for the production of fibers from phenoplast resins of the resole type.
2. Description of the Prior Art
The formation of fibers from phenoplast resin is at present a complicated technique having relatively long stages which are difficult to carry out. These techniques are nevertheless employed because they enable products to be obtained which have remarkable fire resistance characteristics.
The phenoplast resins are obtained by the polycondensation of a phenol and an aldehyde. The most commonly used phenoplasts are obtained by the condensation of phenol and formaldehyde. In the description given below, reference will be made mainly to these phenoplast resins based on phenol and formaldehyde, but the characteristics of the invention enable it to be applied to any phenoplast resins, provided they have the properties indicated hereinafter.
Phenoplast resins are conventionally divided into two groups known under the generic terms of "novolaks" and "resoles". These terms serve to distinguish products which differ substantially from one another in their method of preparation, structure and certain properties.
As a simplification, the novolaks may be described as being obtained by a polycondensation in which the phenol is used in excess of the formaldehyde in the presence of an acid catalyst. The resin obtained, which is thermoplastic, may be cross-linked by means of a cross-linking agent such as hexamethylene tetramine or paraformaldehyde in the presence of an acid catalyst. Cross-linking is accelerated by elevation of temperature.
Stated in simplified form, the resoles may be regarded as being obtained by a polycondensation in which the formaldehyde is used in excess of the quantity of phenol, in the presence of an alkaline catalyst. Formation of the resin, which is accelerated by the elevation of temperature, is difficult to control. The end products obtained vary widely according to the operating conditions employed and in particular the duration of the reaction. If the reaction is not stopped, it continues to the formation of a solid product which is infusible and therefore cannot be spun or drawn out. In order to maintain the resin in a workable condition, the reaction should be stopped by a lowering of the temperature and/or the neutralization of the mixture. The resin is then in the form of a solution whose characteristics, viscosity in particular, vary widely according to the degree to which the reaction has progressed. The resin is capable of being cross-linked and such cross-linking may be accelerated in the presence of an acid catalyst. The speed of cross-linking increases with rising temperature.
In practice, only phenoplast resins of the novolak type are at present used for the production of fibers, no suitable techniques being known for the production of fibers from resoles.
Novolak fibers are conventionally produced by melting the thermoplastic resin and then fiberizing the molten resin and treating it with the cross-linking agent and catalyst in an aqueous or gaseous medium.
This treatment resulting in cross-linking is very lengthly since it requires the cross-linking agent and the catalyst to diffuse into the fiber of solidified resin, and it may extend over several hours.
It has been proposed to speed up the treatment by forming the fibers from a mixture of the molten novolak resin and the cross-linking agent. However, the process of cross-linking in an acid, gaseous phase at an elevated temperature under pressure, which in this technique takes place after fiberization, is a delicate operation and difficult to carry out as a continuous process such as is necessary for the production of large quantities under economic conditions.
In the case of resoles, the operation resulting in formation of the fibers in particularly delicate. In contrast to novolaks, for which cooling after passage of the molten mixture through the bushing results in fibers which have to some extent solidified and been individualized even if cross-linking has hardly begun, fiberization in the case of a resole in a state suitable for spinning, that is to say a resole whose reaction has been stopped at a degree of condensation corresponding to a suitable viscosity, results in the production of fibers which are not stabilized but remain glued together.
The invention proposes to provide a process for the production of fibers from resoles.
To achieve this purpose according to the invention, the resin used and the nature and proportions of any added products, in particular a cross-linking catalyst, are chosen to form a mixture whose characteristics, in particular its viscosity, are suitable for the formation of fibers by passage of the mixture through a bushing.
The composition to be fiberized, in which the viscosity conditions may have been adjusted by the addition of solvents, is immediately conducted towards an apparatus comprising a centrifuge and serving as a bushing. The composition introduced into the centrifuge covers the internal peripheral wall of the centrifuge. This wall is perforated by orifices through which the composition passes. The composition is projected from the orifices in the form of fine filaments which are attenuated into fibers and the dimension of the orifices is chosen so that each of them forms a single filament. The conditions determining the kinetics of maturation of the fibers formed, in particular the choice of catalyst and possibly also of the proportions in which it is used, and the temperature conditions of the surrounding atmosphere into which the fibers are projected, are chosen so that the fibers become sufficiently cross-linked and dried in the course of their path through this atmosphere to the apparatus receiving them so that they maintain their own form and do not stick together.
One of the main difficulties encountered in the formation of fibers from resoles is connected with the fact that it is necessary to use highly unstable compositions. This problem does not arise in the case of novolaks. The thermoplastic character of novolaks enables the formation of fibers to take place quite separately from the cross-linking of the resin. In the production of novolak fibers, the stability of the resin is used to advantage. In the case of resoles, the compositions in solution do not allow the two operations to be separated and the formation of fibers must therefore take place at the same time as the processes of cross-linking and drying which lead to the formation of "stabilized" fibers.
The term "stabilized" is used in the present description to denote fibers which are sufficiently developed to enable them to preserve their own form even if they have not yet attained the final mechanical properties of the completely cross-linked fibers. Moreover, their surface condition is such that they are not liable to stick together when they are gathered together and therefore in contact with one another. The "stability" of the fibers is, of course, related to the conditions under which they are prepared. In the course of this production, the fibers are subjected only to limited mechanical stresses, as will be seen hereinafter.
In other words, the preparation of resole fibers is subject to contradictory requirements. On the one hand, it would seem desirable to prepare a mixture capable of accelerating the process of development while on the other hand, if such a mixture is effectively obtained, it is difficult to control the development of the mixture sufficiently to keep it under conditions suitable for its passage through a bushing and attenuation of the fibers.
In order that the compositions used according to the present invention may be fiberized and bearing in mind their rapid development towards a state in which they can no longer be used for the formation of fibers, it is necessary to arrange for the formed mixture to be used very rapidly.